Here is an informative video on AR-15 barrel profiles, length and fluting. There are likely some things in here you didn’t know, as was the case with me. Even if you’re a gunsmith. To some degree this is “gilding the lily.” Most AR-15 barrels are just fine for civilian applications, even in situations of duress like 3-gun competitions. Also, the military has made some bad decisions before on this subject. Imagine that. But there are some bad decisions you can make, and it pays to know what he teaches you.
Nathanial from Faxon Firearms delves into barrel profiles, barrel lengths, fluting and finalize some thoughts on nitride.
Next up Brownells On carbine buffers.
Finally, as a bonus for the avid AR-15 lover like me, this is yet another good video on the operation of the gun in semi-auto and fully automatic mode.
There are a lot of articles and discussion forum threads on barrel twist rate for AR-15s. So why am I writing one? Well, some of the information on the web is very wrong. Additionally, this closes out comment threads we’ve had here touching on this topic, EMail exchanges I’ve had with readers, and personal conversations I’ve had with shooters and friends about this subject. It’s natural to put this down in case anyone else can benefit from the information. Or you may not benefit at all. That’s up to you.
This is a discussion about 5.56mm ammunition and barrel twist rates (and later, about the shooter and ammunition quality). If you wish to debate the effectiveness of the 5.56mm round generally, or wish to disparage the choice of the Eugene Stoner system, I’m sure there are forums for you. This is not it.
In the real world, ammunition isn’t concentric, and even if it is almost precisely concentric, pour density can be slightly different throughout the ball, and voids can develop. This causes gyroscopic stability problems with bullets, even in the best manufactured ammunition. But much ammunition would not be considered the “best manufactured ammunition.” Ammunition will only be as good as the QA under which it was made.
When center of gravity is off-axis it can cause bullet lateral throwoff, yaw and a host of other problems with bullet trajectory. In order to overcome these problems, rifling twist achieves this gyroscopic stability for the bullet, thus negating the effects of the manufacturing process (at least in part).
Conventional wisdom taught us that slower twist rates wouldn’t properly-stabilize a bullet, causing it to yaw. On the other hand, faster rates could over-stabilize lighter bullets, causing similar problems. This is correct in theory—however, modern ballisticians have pretty much de-bunked the over-stabilization theory as a practical matter. All things being equal, it is better to have too much twist than not enough.
While his statement is a bit imprecise, there is something very precise about it. It is precisely wrong. Yet there are much cleaner and simpler explanations of why high twist rate is not always good. One commenter at this discussion thread summed it up well.
You can certainly overstablilze (sic) a bullet if you spin it so fast it doesn’t nose over at the top of its trajectory … Best thing to do is not spin bullets any faster than what’s needed for best accuracy.
Correct. If a bullet is overstabilized, it tends to stay pointed along its axis of rotation, even on the final (downward) part of its trajectory. This can cause keyholing, odd aerodynamic effects (flying sideways through the air) and even bullets to wildly spin off trajectory.
Bullets from rifled barrels eventually achieve stability by yawing back and forth, while undergoing a larger revolution about the major axis of the trajectory. So quite obviously, it’s necessary to spin the bullet, and to spin it enough to give it stability, while protecting the need to nose over on the final part of its trajectory. Getting this twist rate and spin right has been a matter of much testing, internet fights, and lot of engineering study and heavy spending by the taxpayers. I know that my guns perform well, and so I decided to contact my manufacturer for his opinion on the matter.
In the interest of full disclosure, I have two Rock River Arms rifles, one Elite CAR A4 with a 16″ Barrel, twist 1:9, Quad Rail, and another competition gun with a muzzle brake and 18″ SS barrel with a twist rate = 1:8. I have recommended RRA rifles to my readers before, but there are many good guns on the market. Your probably have one. I sent a list of three questions to RRA, and Steve gave me these responses (the question isn’t included because it wasn’t forwarded back to me, but it’s apparent what I asked except for the first question, which was basically does RRA warranty their 1 MOA for both M193 and M855. This is Steve’s response.
Herschel,
Thanks for your questions. I’m going to take them in reverse order.
3. 1:9 is adequate for many, but not all rounds typically used in an AR platform. Between .223 Remington and 5.56mm NATO, there are rounds from 45 to 90 grains (that I am familiar with) and I know of, but have never shot, lighter and heavier rounds. No single twist is going to handle all of them. 1:9 is adequate for a sizable number of them, however…including the two most commonly available, in bulk and at reasonable prices…55gr FMJ (M193)and 62/63gr FMJ (M855). It is not ideal for rounds lighter than 50gr nor those over 68 or 69 grains, which is why there are other twist rates commonly available…including from RRA. We offer a 1:12 24” bull barrel for our Varmint hunters who prefer to use the lighter bullets for prairie dogs and other targets, and both 1:7 and 1:8 barrels in a variety of configurations for those who want to shoot heavier bullets…up to and including the newer 77gr loads and 80gr VLDs. We’ve also run custom twists for a limited number of contracted purchases.
2. Yes. 1:9 does well with both M193 and M855. Different barrels perform differently, but 1:9 generally stabilizes both weight/length bullets fairly well, It neither over nor under spins either and does not produce key holing.
1. The hardest question to answer. Neither M193 nor M855 are notoriously accurate rounds. They meet military, not match, requirements. Our accuracy claims are the rifle’s capability…but the shooter and ammo have to do their parts. There are loads that are commercially available and claimed to be “M193” and “M855” equivalents that clearly aren’t, and they aren’t capable of ”minute of bad guy” at 100 yards, let alone the .75 to 1.5 MOA claims that we make for our different rifles. That is no reflection on our rifles or barrels, or the shooters…unfortunately there is some real crappy ammo on the market today, which will not perform well out of any barrel, of any twist rate.
Thanks.
Steve/RRA
This is a good response, but let’s not stop here. While perhaps not recalled by some, American Rifleman has given us a fairly comprehensive look at 5.56mm ammunition and barrel twist rates in an article entitled Testing The Army’s M855A1 Standard Ball Cartridge. It is rich with history on how the Army fielded the M855A1. Ignore the issue of the M855 versus the M855A1 for a moment and consider the background.
Accuracy cannot be assessed without addressing the rifle barrels’ twist-rates. In the early 1980s the M855’s 62-grain bullet was developed for the M249 Squad Automatic Weapon (SAW). For purposes of interoperability, the same load was adopted as the M16A2 rifle’s standard ball as well. A February 1986 U.S. Army study noted that the M855’s bullet required a “1:9 twist [which] would be more appropriate for the M16A2 rifle, improving accuracy and reliability.” Multiple studies confirmed the 1:9-inch twist requirement.
But then a problem arose. The U.S. military’s standard M856 5.56 mm tracer round was longer, heavier (63.7 grains) and slower than the M855 ball, and simply would not stabilize with a 1:9-inch twist barrel. Thus, despite it doubling M855 group sizes, the M16A2 (and later, the M4) specified a 1:7-inch rate-of-twist barrel to stabilize the tracer round. It remains so to this day. Therefore, M855A1 was test-fired with both 1:7- and 1:9-inch twist barrels, and it was verified that this new cartridge is consistently more accurate in the latter barrels-as was its predecessor.
Don’t slip past these paragraphs, because they explain why “Milspec” is 1:7. It isn’t because 1:7 shoots M193 or M855 more accurately. It’s because of the weight of tracer rounds. As we’ve discussed before, the term Milspec doesn’t mean better, or worse, or anything at all except that it precisely meets the specifications outlined in the purchase order(s), excepting whatever variance notifications they might make on a given batch of guns.
The M855A1’s developers have described it as yielding “match-like” accuracy, which most rifle shooters would define as one minute-of-angle (m.o.a.), or groups measuring no more than 1 inch at 100 yards. While the new ammunition has proved more accurate than the green-tipped load it replaced, testing did not yield match-like accuracy, especially in the standard 1:7-inch twist-rate found in today’s M4s and M16s. At 100 yards, the best group with a 1:7-inch barrel was 1.62 inches (1.6 m.o.a.). At 300 yards. it similarly fired 1.6 m.o.a. (4.9 inches) and widened to 1.8 m.o.a. (7.5 inches) at 400 yards. At these same distances, firing the M855A1 through a 1:9-inch twist barrel reduced group sizes by approximately half.
The tests demonstrated that 1:9 twist produced better accuracy, approximately twice as accurate. Now take note what the testers found with the newer M855A1 regarding repeatability.
On average, the new ammunition produced one flyer in roughly each five rounds, which, it can be argued, exaggerated the group sizes. Since the Army announced that, “On average, 95 percent of the [M855A1] rounds will hit an 8×8-inch target at 600 meters,” each group’s most errant bullet impact was discarded and group sizes recalculated. Statistically they improved, but not enough to place 95 percent of rounds so close at 600 meters, at least when using the standard 1:7-inch barrel-which may explain why accuracy was less than expected.
There is one “flyer” in every five rounds. This seems to me to be a significant problem with this ammunition combined with the barrel twist, and the commenters don’t seem to like it very much either. Finally, this.
When U.S. Army shooters twice fired public demonstrations of the new round, they did not employ standard 1:7-inch twist M16A2s or M4s, but accurized, match-grade, stainless-barreled rifles from the Army Marksmanship Unit (AMU). I contacted the AMU and learned that these rifles did not have standard-issue 1:7-inch barrels, but most likely 1:8-inch twist, which probably accounts for their “match-like” accuracy.
Isn’t that rich? The Army made claims of “match-like accuracy,” and proved the rounds shooting out of different barrels than are deployed with Soldiers, using 1:8 twist, not 1:7 twist.
The American Rifleman article goes on to discuss in some detail the performance of the M855A1 with slim-profiled targets like malnourished tribal fighters in Afghanistan (so-called “ice picking” the target without fragmentation), performance at barrier penetration (concluding that it is better than its predecessor), and its lethality once it does penetrate barriers. I recommend this reading to you. It’s well worth the time.
So to summarize what we know, remember some basic things. First, the bullet has to be spun to give it gyroscopic stability. This spin needs to match the bullet (including mass and length), and care must be taken not to over-stabilize the bullet. If you shoot typical .223 ammunition (55 gr.), or M193 or M855, a twist rate of 1:9 is probably just about ideal. You’ll probably lose some accuracy with a higher twist rate.
This loss of accuracy is likely not significant for a lot of shooters. If you shoot much heavier ammunition (and there is a lot on the market), you probably need to consider a twist rate of 1:8. Finally, none of this matches the value of good ammunition or good shooting.
That’s the good news. Most guns can outperform the shooter, and I know that’s the case with me. I’m a decent shooter. Not great, but decent. I’ve taken my Tikka T3 .270 bolt action rifle and literally put rounds through the same hole at 100 yards (with slightly more tearing of the same hole in the paper). On the other hand, this is with a good scope, no wind, a cool and comfortable day, all day to work my craft and thus no time pressure, no one else to be concerned about, lots of coffee to wake up, and a full belly.
But if I had kept records, it wouldn’t have happened again exactly like that since, theoretically, even with perfect ammunition, considering barrel harmonics and that physical processes like this are a heuristic phenomenon, if I had continued to log my shots this way, it would have doubtless shown a standard distribution (distance between each shot and mean).
But regardless of the details, you’ve done it before. Control breathing … get good sight picture … back out of the shot if you’re not mentally right … know where your trigger breaks … and so on. You know the drill, since you’ve done it many times. It’s perhaps the purest pleasure a shooter can have.
Now throw in simple annoyances like a whining partner at the range, losing daylight and time pressures, hunger, and any of the other 100 possible nuisances that can sap your accuracy. Then your accuracy goes to hell, doesn’t it? Now, combine that with wearing heavy gear and being shot at, and I’m sure it diminishes your control over your weapon. Thankfully, I only have the experiences of my former Marine son conveyed to me.
The good part of this is that regardless of your barrel twist rate, if your AR-15 is reliable, even if it’s not top of the line, it can probably outperform you. That means getting better isn’t a matter of getting a new rifle or barrel with a different twist. It means practicing with your rifle, sometimes under duress. It also means buying good ammunition. Steve at RRA is right. The shooter and ammo have to do their part. I object to cheap ammunition just like I object to cheap engine oil. I’m trying to develop the discipline at the store or online to buy better ammunition.
Right, I’ve got it. I feel your objection. Good ammunition (e.g., Hornady $2 per round .270 for my Tikka) hurts. This is my wealth, and it’s hard to part ways with it since it’s hard to earn it. But using bad ammunition at the range makes it hard to impossible to assess your practice. Use of my value pack Federal .223 at the range means that my accuracy is irrelevant if I’m using the same reticle holdovers I would for 5.56mm since the muzzle velocity is different (and very slightly lower than the 5.56mm). You’ve got the picture.
The best way to get better accuracy is probably not to get a better gun. It’s to practice with the one you’ve got.
Here is a related video I found interesting on gyroscopic stability. He’s wrong about the math being incomprehensible, but it is rather difficult if you’re involved with partial differentials or worse, the Navier-Stokes equations in CFD. You need some specialized training in mathematics in order to tackle that. You don’t have to know any of that in order to understand the basics of shooting.
This discussion probably won’t end the debate on barrel twist rate, and it certainly won’t end the fight between the Army and Marine Corps (who doesn’t want to deploy the M855A1). But I hope it was helpful to you.
I can no longer place any confidence in the AR-15 to defend myself in a time of disaster or urban unrest. To protect myself and my family, I have completely switched over to the AKM family of weapons—a semi-automatic variant of the legendary AK-47. I know that somewhere, someone is saying out loud that I am a dumb-ass or a communist. When I first discussed this with one of my close friends, a career Army infantryman with more than 16 months of combat duty spent with the 172nd Infantry Brigade, he just shook his head in disagreement. I remember telling him over some beers, and he just kept asking “Why?” with a look of disappointment and bewilderment on his face.
The M16A2 was my primary weapon for my entire career in the United States Air Force; I knew it inside and out. I was one of the few USAF engineers I knew who loved target shooting, going to the range, hell, I even liked to clean guns. So why walk away from a weapon platform I had used or owned for over 20 years? The answer comes down to two major reasons: supportability and simplicity.
Any military or company that uses AR-15s/M4s has much deeper pockets than I will ever have. Major corporations and military units typically have a robust supply system that can provide an individual any desired replacement part they may need. This is necessary, because the variation in parts from manufacturer to manufacturer is immense, making interchangeability difficult.
The buffer spring and buffer weights alone have more than a dozen different variations depending on barrel length and number of coils on the buffer spring. Then, stop and consider the rifling twist rates in the weapon’s barrel and what projectile works optimally in them. Here is a sample of the variations and options on buffer weights alone.
[ … ]
The word simplicity might cause people to think that the AR-15/M4 is a difficult weapon to operate, and that is not the case at all. When I use the word simplicity, I think of overall use and maintenance of the weapon. How maintenance-intensive is it? What type of lubricant do I have to use for my situation?
With the AR-15/M4, there is an entire segment of the shooting world that will launch into heated arguments about what lubricant works best. I have seen the debates between CLP versus Frog Lube versus Fire Clean. Running the gun ‘wet’ or ‘dry’ is another topic of debate. I have even heard people who say they use WD40 or Castrol Slick 50 on their AR-15s/M4s.
Properly lubricating a weapon isn’t rocket science, but improper lubrication can lead to the weapon jamming at the most inopportune moment. In the case of over-lubrication, in a dusty or sandy area, excessive dirt and debris can accrue on the weapon’s working parts, causing stoppages and malfunctions.
Consider my selection. The AKM can be lubricated by just about anything that has some degree of viscosity. Bearing grease, lithium grease, motor oil, 80W 90 gear oil, the tears of liberals, the blood of unicorns, you get the idea. There is a scene in the video below (at about the 20-second point) where you can see an AK-47—buried in the dirt and clay of Africa for 18 years—rendered operable with only a can of motor oil.
Good grief.
And AR-15s can be made to function with a dab of motor oil too. And he acts like there’s no such thing as AR-15 stress tests out there on YouTube. I’ve linked and embedded so many they’ve almost become boring to watch now.
So it all boils down to this after reading this tiresome and stolid article. The author doesn’t like to think about anything, doesn’t like variability, doesn’t like to be able to modify his gun, doesn’t like variants on a theme, and doesn’t want to have to worry about the engineering mechanics behind the rifle or proper selection of components. Therefore, he has found himself some canned excuses to jettison the AR-15. He’s lazy.
Eugene Stoner is unimpressed. I am too. If this is the best that web site can do, I won’t be returning. This piece is not even a thinker piece that makes you ponder the more complex issues behind gunsmithing or engineering. It’s completely un-compelling.
The telltale sign that it’s going to be a bad article was in this sentence: “I was one of the few USAF engineers I knew who loved target shooting, going to the range, hell, I even liked to clean guns.”
So there you have it – what a USAF “engineer” thinks about guns. Stick to airplanes. Go back to something you’re good at. I guess.
“Why I abandoned the AR-15” are words you’ll never see here at this web site except in mockery.
We have previously discussed the Marine Corps battle with the Army and Congress over fielding a different ammunition than the Army, who spent a wad of money on an “environmentally friendly” lead-free cartridge, the M855A1.
Currently, the Marine Corps is trending towards the MK 318, which appears to be a far superior round, and it comes in right at 2900 FPS out of the M4 barrel, higher for longer barrels. The claim is that it behaves better at longer distances and retains its ability to penetrate.
This trend towards heavier rounds has been going on for some time now, and 62 grains isn’t the top weight for the 5.56mm bullet. One reader sends information about Sierra 77 grain, and tells me that the 1:9 twist is just fine with this ammunition. Of course, one gives up something to get something. In the case of heavier bullets, you give up muzzle velocity.
This velocity detriment may seem small. TFB likes the Sierra 77 grain, and informs us that its muzzle velocity comes in somewhere between 2500 FPS and 2600 FPS. But your choice of ammunition will depend upon your target, its distance, any interstitial shielding, potential body armor, etc.
You may do better with M193 than with either the MK 318 or the Sierra 77 grain. Sometimes the smaller rounds with the higher muzzle velocity are what’s needed to penetrate any armor. Do you not believe me? Consider what we learned with the FN 5.7 and its test against bulletproof glass, which only the .454 Casull could penetrate. The open tip 5.7 round at 22 grains penetrated the glass due to high muzzle velocity, whereas the heavier 5.7 round did not.
Do you need more evidence? Very well. Consider that AR500.com sells hard plates it calls Level III, and those plates are rated to stop M855 (steel core) but cannot stop M193. They have to move up to what they call Level III+ to perform effectively against the M193 due to its higher muzzle velocity compared to the M855. There’s nothing wrong with having a safe full of M193.
Tom McHale has a very good article at Ammoland on AR-15 rifle barrel lengths and whether it matters.
Remember that you have two competing effects on muzzle velocity. First, it’s advisable to get as much work out of expanding gases as feasible in order to increase muzzle velocity. Second, there is friction in the barrel, which is a detriment to the work being done by the expanding gases.
So there is a turnover point on the curve of barrel length versus muzzle velocity, where you no longer gain muzzle velocity with increased barrel length. So McHale performed some testing of barrel lengths, and this is what he came up with.
The difference between the 16″ and 18″ barrel is greater than the difference between the 18″ and 20″ barrel. But barrel lengths greater than 18″ doesn’t buy you much. McHale also has some data on the .300 Blackout round that looks interesting.
The one thing he didn’t give us is the effects of 14.5″ barrels (as with the M4), or pistol length barrels (e.g., 10″ barrels). I would like to see some test data on that, and unless persuaded otherwise I have to believe that the SpecOps trend to use shorter and shorter barrels lengths along with suppressors is adversely effecting muzzle velocity.
RTH Firearms today released its first semiautomatic production rifle chambered in 6.5x47mm Lapua or 6.5mm Creedmoor precision cartridges.
These rifles achieve amazing precision, hitting below sub-MOA at 100 yards with the ability to reach 1,400 yards with very little felt recoil.
Here is a picture of it.
Sweet rifle, yes? Here is the web site, and here is the URL for the gun. Now get ready for the sticker shock. Are you ready?
$6,899.99.
We covered precision shooting and semi-automatic mid-range caliber rifles before, and I noted that the guns were in that price range as well, most of them special order guns. They’re not shelf guns, they’re not even mid-range priced. These aren’t even highly priced, as you can get designated marksman rifles that will shoot < 1 MOA for much less than that.
I’m sure this gun is nice, but folks, what good is it, realistically, to have a gun that shoots < 1 MOA as compared to a gun that shoots 1 MOA, unless you’re a professional precision shooter or a professional sniper in the military? And not only that, what good is it to have a gun that can shoot better than you can?
I admitted to my failures several weekends ago, failures that had to do with not checking my dope before I started shooting, and failing eyesight forcing me to consider a higher power scope. Perhaps with perfect eyesight (uncorrected), years of experience, and your livelihood depending upon accuracy, it makes sense to worry over this sort of thing.
I suppose that they are targeting the very people I said would be interested in this gun, rather than the typical man who has trouble affording even much less than that, even for a really nice AR. It would be nice to have a mid-range caliber AR that shoots well and doesn’t tear the gun up (like I hear about AR-10s and the .308), but until manufacturers are able to pull the price down by a wide margin, this isn’t going to happen for the common man. They won’t be making many of these guns.
In the mean time, it’s easy enough to take my really nice ARs and slap a .300 Blackout upper receiver on it (which takes 10 seconds) and have a really nice mid-caliber AR that shoots 1 MOA or slightly less, which is as good as I can shoot anyway.
The main conclusion is that this is a decent budget AR. He put it through what looked like a fairly comprehensive torture test, although I don’t know how many AR-15s or what brands he has put through the test.
7 years, 11 months ago
